The general laws governing rotary pumps suggest that the stream of fluid conveyed by the pump increases in proportion to the speed of the pump, the conveying rate increases by the square of the speed and the pump output increases by the cube of the speed. These laws accordingly suggest that rotary pumps should be driven at high speeds. Since a pump unit can be driven at different speeds, a whole range of different speeds can be achieved with one unit without constructional modification thereof. Thus by appropriate choice and control of the speed of the pump, a pump unit can be run for optimum economy.
Control of the speed of a pump unit of the type described above is achieved by means of the frequency converter.
By virtue of developments in the field of electronics frequency converters can now be made of such small size as to be capable of integration into a pump unit of the type concerned. Such a pump unit having an integrated frequency converter is described, for example, in German Patent Application No. 36 42 727, which discloses an underwater motor pump having a built-in frequency converter. The motor of the pump is a wet rotor motor, that is to say the rotor runs in a can flushed through by the conveyed fluid. The conveyed fluid flowing through the can is used for cooling both the motor and the frequency converter. A disadvantage of such wet rotor motors resides however in huge frictional losses by the rotor running within the fluid-filled can. The efficiency of the unit is, therefore, drastically reduced, especially at higher speeds.
In pump units, for example, in swimming pool technology, the rotor of the electric motor is sealed from the conveyed fluid and thus suffers considerably less frictional losses than a wet rotor. In this case the motor is cooled by means of a cooling jacket on the outer circumference of the motor stator, through which jacket part of the stream of the conveyed fluid passes. A motor for such a pump unit is disclosed for example in German Patent No. 37 38 592. When such a motor is controlled by a frequency converter, the frequency converter must regularly be cooled. Since for air cooling a frequency converter, a large convection cooler is needed, the size of the pump unit is substantially increased. Although for liquid cooling, no large cooling means are needed, the cooling liquid, as a rule the conveyed liquid, must be supplied by way of pipes to the frequency converter and removed therefrom, which is uneconomical.